1. Field of the Invention
This invention relates to a steel material of high fatigue strength in the form of a slab, or a like material prepared by continuous casting which can be hot worked to make a tube, round bar, sheet, etc., and a process for manufacturing the same.
2. Description of the Prior Art
A steel slab, or a like steel material prepared by continuous casting is usually used for making a tube, round bar, sheet, etc., but the removal of its skin by cutting, grinding or pickling for processing is likely to result in a product having non-metallic inclusions exposed on its steel surface or remaining in a layer immediately below its surface, or having small cracks remaining or formed in its surface. The non-metallic inclusions are mainly oxides, such as Al2O3 and CaO, having a very high degree of hardness and a very low degree of elongation as compared with steel. Most of the non-metallic inclusions and cracks have a maximum size of, say, 20 to 30 microns.
Those non-metallic 20-30 microns inclusions and cracks occurred on the surface of the steel or therein lower the fatigue strength of a steel tube, bar, sheet, etc. when a fatigue load has acted upon it in an environment causing its fatigue, since they concentrate a fatigue stress and start its fatigue failure. I, the inventor of this invention, have, however, found that the non-metallic inclusions are responsible for the fatigue failure of a steel material having no crack at all, or having only so small cracks as not to cause its fatigue failure. The fatigue failure test which I conducted on a steel material having a surface worked to leave only cracks as small as up to, say, 5 microns, gave a fracture showing non-metallic inclusions having a size of, say, 20 to 30 microns, and I have concluded that those inclusions caused the fatigue failure of the material. It is considered that a reduction caused by non-metallic inclusions to the fatigue strength of steel is due to an increased concentration of stress caused by a difference in hardness and elongation between the steel and non-metallic inclusions.
Under these circumstances, it is an object of this invention to provide a steel material having such a high fatigue strength as to withstand use in an environment by reducing the influence of the non-metallic inclusion on its surface, or surface layer as much as possible by means of hot working, such as forging, rolling or extrusion, or its skin removal and others.
It is another object of this invention to provide a process for manufacturing a steel material of high fatigue strength.
These objects are attained by a steel material having a layer of increased fatigue strength formed by diffusion under heat from a nickel plating layer on a surface formed by hot working, such as forging, rolling or extrusion, and skin removal, and containing non-metallic inclusions therein, or in a layer immediately thereunder, and by a process which comprises the steps of plating with nickel a surface in or under which steel contains non-metallic inclusions, and heating the nickel to cause it to diffuse into the steel surface to form a layer of increased fatigue strength.
When a steel material prepared by hot working, such as forging, rolling or extrusion, has at least a part of its surface subjected to skin removal treatment as required for the manufacture of a final product, the non-metallic inclusions which it contains are exposed in its surface, or come to stay in a layer immediately thereunder. According to this invention, therefore, the surface in or under which the steel material contains such non-metallic inclusions is plated with a nickel layer, and the nickel layer is heated to cause the diffusion of nickel to form a surface layer of increased fatigue strength. The skin removal treatment may be carried out by any appropriate method, such as cutting, grinding, polishing, pickling, chemical polishing, electrolytic polishing or melting.
The layer of increased fatigue strength is higher in hardness and lower in elongation than steel, and as the non-metallic inclusions and the steel surface have a smaller difference in hardness and elongation, the steel surface has a lower degree of fatigue stress concentration and thereby an improved fatigue strength. The layer is not particularly limited in thickness, but preferably has a thickness of, say, 10 to 30 microns. It makes the steel material highly reliable for use even in an environment creating a large amount of fatigue stress.
The process of this invention can be carried out by using any existing equipment and thereby makes it possible to manufacture a steel material of high fatigue strength at a low cost.
The invention will now be described in further detail by a few specific examples.